September 18, 2013

Avian Evolution: How Raptor Limbs Became Bird Wings

Scientists are certain that sometime around 150 million years ago birds originated from a group of small, meat-eating theropod dinosaurs called maniraptorans. According to recent studies conducted around the world, the maniraptorans were very bird-like, with feathers, hollow bones, small body sizes and high metabolic rates. What remains unclear is at what point the forelimbs evolved into wings, making it possible for the maniraptorans to fly.

To answer that question, McGill University professor Hans Larsson worked with former graduate student Alexander Dececchi to examine fossil data, greatly expanded in recent years, from the period marking the origin of birds. Their findings, published in the journal Evolution, revealed that throughout most of the history of carnivorous dinosaurs, limb lengths showed a relatively stable scaling relationship to body size.

This scaling relationship remains stable despite the 5,000-fold difference in mass between the smallest feathered theropods from China to the Tyrannosaurus rex. At the origin of birds, however, this limb scaling changed. Both the forelimbs and hind limbs underwent a dramatic decoupling from body size - which might have been critical in allowing early birds to evolve flight, and then to exploit the forest canopy.

The lengthening forelimbs became long enough to serve as an airfoil. This allowed the evolution of powered flight. The shrinking hind limbs combined with the lengthening forelimbs to refine flight control and efficiency in early birds. These shorter legs would have aided in reducing drag during flight. This is the same reason modern birds tuck their legs up as they fly. Perching and moving about on small branches in trees would also be facilitated by shorter hind limbs. At a time when pterosaurs, another group of flying reptiles, dominated the skies and competed for food, the combination of better wings with more compact legs would have been critical for the survival of birds.

"Our findings suggest that birds underwent an abrupt change in their developmental mechanisms, such that their forelimbs and hind limbs became subject to different length controls," says Larsson, who is also the Canada Research Chair in Macroevolution at McGill's Redpath Museum.

A major shift in function or behavior is usually indicated by deviations from the rules of how an animal's limbs scale with changes in body size - for example, the relatively long legs and short arms of humans. Larsson adds, "This decoupling may be fundamental to the success of birds, the most diverse class of land vertebrates on Earth today.."

"The origin of birds and powered flight is a classic major evolutionary transition," says Dececchi, now a postdoctoral researcher at the University of South Dakota. "Our findings suggest that the limb lengths of birds had to be dissociated from general body size before they could radiate so successfully. It may be that this fact is what allowed them to become more than just another lineage of maniraptorans and led them to expand to the wide range of limb shapes and sizes present in today's birds."

"This work, coupled with our previous findings that the ancestors of birds were not tree dwellers, does much to illuminate the ecology of bird antecedents." says Dececchi. "Knowing where birds came from, and how they got to where they are now, is crucial for understanding how the modern world came to look the way it is."